1. Field of the Invention
The present invention relates to a sheet feeding apparatus for the feeding of sheets such as originals or recording sheets to an image forming apparatus, such as a copier or a printer. In particular, the present invention pertains to a sheet feeding apparatus that can sequentially feed sheets so that the sheets overlap each other a constant amount, or so that there are no intervals between the sheets.
2. Related Background Art
Generally, when recording sheets are overlapped even slightly and conveyed to an image forming apparatus, the apparatus can not satisfactory perform image forming. Therefore, to prevent the double feeding of recording sheets, some image forming apparatuses include sheet feeding devices that convey recording sheets separately and at predetermined intervals.
As is shown in FIG. 16, this sheet feeding apparatus includes a pickup roller 100, for feeding recording sheets P stacked on sheet stacking means (not shown). When sheet feeding is stated, the pickup roller 100, which is generally positioned above the topmost recording sheet, descends until it abuts against the upper surface of the topmost recording sheet P, and then employs a rotational friction force to feed the recording sheet P to nips between sheet feed rollers 102 and retard rollers 104.
The recording sheets P can be regular paper, thin resin sheets used as substitutes for regular paper, postcards, cardboards, envelopes, or thin plastic sheets.
After the pickup roller 100 has fed the recording sheet P to the nips between the sheet feed rollers 102 and the retard rollers 104, it is raised and suspended above the recording sheets P, ready to feed the next (succeeding) sheet. The recording sheet P that was fed out is conveyed while it is pinched between the sheet feed rollers 102 and the retard rollers 104.
When the trailing end of the preceding recording sheet P passed under the pickup roller 100, the pickup roller 100 descends and feeds the next recording sheet. In order to precisely feed the next recording sheet P in this case, the pickup roller 100 descends to feed the next sheet a slight time period after when the trailing end of the preceding recording sheet passes under the pickup roller 100.
If a plurality of recording sheets P are fed out by the pickup roller 100, the recording sheets other than the topmost one are returned to the sheet stacking means by the retard rollers 104.
FIG. 17 is a diagram showing the arrangement of the separation portion (unit) of the sheet feeding apparatus. When sequential paper feeding is performed with this arrangement, at a specific time, immediately before the trailing end of the recording sheet conveyed by the sheet feed rollers 102 and the retard rollers 104 passes through the nips N, between the sheet feed rollers 102 and the retard rollers 104, an electromagnetic clutch 110 is turned off to cut the input of drive (driving power) to a sheet feed roller shaft 102a .
After the input of the driving power is cut, the recording sheet P is pulled out while the sheet feed rollers 102 are driven to be rotated by a pair of drawing rollers 106 that are located downstream of the sheet feed rollers 102. In FIG. 17, a torque limiter 108 idles upon the application of an amount of torque equal to or greater than a predetermined amount and thus prevents the transmission of the torque to the retard rollers 104, a one-way clutch 112 is provided on the sheet feed roller shaft 102a to reduce the load applied by the drawing roller pair 106, and a drive input belt 114 transmits the rotation of the sheet feed roller shaft 102a to a retard roller shaft 104a.
In FIG. 17, rotation in the direction in which a recording sheet is fed out is transmitted to the sheet feed rollers 102, and rotation in the direction in which a recording sheet is fed backwards is transmitted to the retard rollers 104. However, when no recording sheet is held between the sheet feed rollers 102 and the retard rollers 104, or when only one recording sheet is pinched between them, the torque limiter 108 idles, and no rotational force is transmitted to the retard rollers 104. Therefore the retard rollers 104 is driven to be rotated with the sheet feed rollers 102 or the recording sheet in the direction in which the recording sheet is fed out. When a plurality of recording sheets are inserted between the sheet feed rollers 102 and the retard rollers 104, the retard rollers 104 are rotated in the direction in which a recording sheet is returned, so that beginning with the bottom recording sheet, all the sheets, except for the topmost one, are returned to the sheet stacking means. In this manner, recording sheets can be separated and conveyed.
Since the one-way clutch 112 is provided in this arrangement, when the driving force input to the sheet feed roller shaft 102a is cut and the recording sheet is drawn by the drawing roller pair 106, the driving force input section is driven to be rotated via the retard rollers 104 and the torque limiter 108, and no work is performed by the separation mechanism. Therefore, a succeeding recording sheet is conveyed while overlapping a preceding recording sheet, i.e., double feeding occurs.
In order to prevent double feeding, another one-way clutch 116 is provided in the direction (conveying direction) in which the drive input side of the retard roller shaft 104a is not rotated forward.
In order to cope with recent requests for higher processing speeds, a current image forming apparatus is so designed that not only is the recording sheet feeding speed increased, but also the interval (hereinafter referred to as a sheet feeding interval) is reduced between the recording sheets that are separately conveyed by separation means constituted by the sheet feed rollers 102 and the retard rollers 104. Further, another image forming apparatus has been provided that has a function whereby, even when recording sheets are fed in more or less a double feeding state, the interval between the sheets can be corrected before an image is transferred to the sheet.
However, in the conventional sheet feeding apparatus, to prevent double feeding, the sheet feed roller shaft 102a is not re-driven until a preceding recording sheet has been completely removed by the drawing rollers 106. And because of the period of time during which the sheet feed rollers 102 are therefore halted, the ability to reduce the sheet feeding interval is limited.
In addition, since the conventional sheet feeding apparatus is so designed that double feeding is completely prevented, it can not feed a recording sheet and a succeeding recording sheet, while overlapping them a predetermined amount. Further, when a sheet feeding interval of 0 or a very small value is set, the sheet feeding interval is not stable, and only limited control of the interval is available.